1. Field of the Invention
This invention relates to the field of ophthalmic lens polishing, and particularly to machines and methods for fining and polishing spectacle lenses.
2. Description of the Related Art
Lenses for certain types of eyeglasses are manufactured using a lens blank which is cast with a completed front curvature, but an unfinished back surface. The front surface is blocked to a metal mandrel using, for example, a low temperature metal alloy or a layer of wax. The blocked lens is typically placed in a lathe which machines a prescription on the back surface of the lens, producing a surface that is either spheric or toric (rotationally non-symmetric) in shape. The lathe invariably leaves machining marks on the back surface that must be removed, either by fining (sanding), and/or by polishing to produce an acceptable surface.
Industry practice is to fine and polish the cut lens with a hard lap that has been pre-machined with major and minor axes of curvature that closely match the lens' prescription; abrasive and soft pads are affixed to the lap to respectively fine and polish the machined lens surface. The lap and lens are placed in a "cylinder machine" with their major and minor axes precisely aligned with each other. A cylinder machine typically sweeps the lens back and forth along its major axis, while oscillating the lap in a complex orbital motion. The lap's pad is rubbed against the back surface of the lens to remove the machining marks, thereby smoothing or polishing the back surface.
To obtain maximum fining and polishing effect, the lap's relative travel along the major axis should be at a maximum as long as the surfaces of the lens and the lap pad remain in constant contact with sufficient force to effect the intended polishing. The ideal range of motion will be different for each prescription.
Cylinder machines typically use electric motors. The rotary motion of a motor is translated to motion along the major and minor axes of the lens through geared transmissions and/or belt drives, together with a variety of mechanical linkages. As a result, changing the motion of traditional cylinder machines requires time consuming, complex mechanical adjustments and calibration. To avoid having to make these adjustments for each lens, a nominal motion is selected to accommodate a wide range of prescriptions. However, the use of a nominal motion gives rise to both inefficient fining and polishing, occasional marring of the lens surface, and in many cases, errors between the final lens prescription and the original prescription written by the patient's doctor. These errors must either be accepted by the laboratory and the patient, or the lens must be scrapped.
A cylinder machine's nominal motion can also be excessive for a given lens--causing the lens/pad interface to separate in some areas and mar the lens surface or cause other unacceptable artifacts, and insufficient for other lenses--which gives rise to unacceptably long processing times, or, in extreme cases, insufficient fining and/or polishing to produce a satisfactory lens. Also, although these motion conversion mechanisms are designed to be as random as possible within the prescribed spherical and cylindrical prescription, certain repetitions are inevitable which can give rise to patterns on the lenses which are cosmetically unacceptable to the patient.